1. Field of the Invention
The present invention is related to differential GPS systems, and more particularly, to a DGPS system employing integrity monitoring.
2. Description of the Related Art
Satellite positioning systems, commonly referred to as global positioning systems or simply GPS, are now well-known in the art. Such specific systems, for example, NAVSTAR-GPS, are rapidly being used for determination of the geocentric position of mobile units, such as water and land vehicles, aircraft and survey equipment, to name a few.
In aircraft, GPS systems are being utilized for navigation, flight control, and air space control. These GPS systems may operate independently, or in combination with, among others, inertial reference systems or attitude heading reference systems in order to provide information during an aircraft flight mission.
Global positioning systems similar to NAVSTAR commonly use a GPS receiver, located on a mobile unit, for receiving satellite information signals transmitted from a plurality of satellites. Each GPS satellite transmits an information signal containing data that allows a user to determine the range or distance between selected GPS satellites and the antenna associated with the mobile unit's GPS receiver. These distances and knowledge of the satellite positions are then used to compute the position of the receiver unit using known triangulation techniques. For example, in the NAVSTAR-GPS system, a mobile unit with a GPS receiver, such as an aircraft, detects a pseudo random code contained in a given GPS satellite information signal and derives therefrom the "elapsed time" or time delay between the transmission of the satellite signal and its reception at the GPS receiver. From this time delay, the GPS receiver derives the range between the GPS receiver antenna and the satellite, sometimes referred to as the pseudo range or pseudo range measurement. Herein, the GPS receiver's position, or the mobile unit's position, generally refers to the corresponding antenna position.
In addition, as part of the NAVSTAR-GPS system, each satellite information signal also contains precise ephemeris data and course almanac data which both describe the corresponding satellite orbital trajectory in earth centered space as is well known in the art. The coordinates of the satellite's orbital position at the transmission time may be derived from either the ephemeris data or the course almanac data. The geocentric position of the satellite may be calculated with a higher degree of precision from the ephemeris data than is possible with the almanac data as is well-known
It should be understood that the mobile unit's three-dimensional geocentric position coordinates are referenced to the World Geodetic Coordinate System. Herein, it should be recognized by those skilled in the art that the World Geodetic System is an earth-centered, earth-fixed geocentric coordinate system, which may be converted to any other coordinate system as required by the user. Sometimes the aforementioned coordinate system is referred to as the WGS84 earth-centered, earth-fixed, rectangular coordinate frame. Herein, the World Geodetic System Coordinates should be presumed, and position refers to this three dimensional WGS84 coordinate system.
In order to determine the position of the GPS receiver unit, a minimum of four satellite signals are required, rather than the expected three. This is so, since the GPS receiver includes a receiver clock which is not as accurate as the atomic clock of the satellites. Therefore, receiving satellite information signals from four different satellites provides a complete solution which permits the correction of any receiver clock error as is well understood in the art. Herein, the corrected receiver clock time is referred to as the receiver time. Thus, if signals from four or more satellites are available to the GPS receiver unit, the geocentric position of the receiver may be determined within approximately one-hundred meters of its "true" geocentric position. Herein, the receiver position derived by the triangulation technique using data from multiple satellites is referred to as the "estimated position". The accuracy of the estimated position of the receiver unit is dependent upon many factors including, among others, atmospheric conditions, selective availability, and the line of sight view of the satellites.
Although the satellite positioning system referred to as GPS is by far the most accurate global navigation system ever devised, its accuracy can be boosted using a technique called "differential GPS", sometimes referred to as "DGPS". DGPS can achieve measurement accuracy better than a meter. Differential GPS has been widely used in surveying applications, and now its use is being developed for aircraft approach and landing applications.
With regard to the latter, GPS systems standards have been developed by the RTCA Inc. (formerally called Radio Technical Commission for Aeronautics) in association with aeronautical organizations of the United States from both government and industry. The RTCA has defined performance requirements for a DGPS system as particularly identified in Document No. RTCA/DO-217, dated Aug. 27, 1993; and for GPS performance requirements for navigation equipment as particularly identified in Document No. RTCA/DO208, dated Aug. 27, 1993, both of which are incorporated herein by reference thereto.
As is well understood in the art, a differential GPS system incorporates a reference or "ground station" which includes a GPS receiver's antenna installed at a surveyed site. The geocentric position of the GPS receiver is known from surveying techniques. The GPS ground station receiver determines pseudo range values between the receiver and a plurality of satellites. Since the position of the satellite is derived from the satellite data associated with the satellite signals, and the position of the receiver is also known, a calculated range value therebetween may be determined for each of the tracked satellites. In turn, the difference between the measured pseudo range value and the calculated range value for each of the tracked satellites may be determined. This difference is commonly referred to as the "differential correction". The differential correction value is essentially the pseudo range error between the "observed" or "measured" pseudo range value derived from the satellite signal travel time and the calculated range value between the antenna's position and corresponding satellite position.
The motivation for differential operation is that many of the largest GPS error sources, such as selective availability and ionospheric effects, are common to two or more receivers operating in spatial and temporal proximity, since these anomalies affect the satellite signal travel time. These error sources can be nearly eliminated in the differential mode, by determining and applying the differential correction value or pseudo range error for greatly enhancing performance. A differential GPS system is shown and described in a publication entitled, "Design and Flight Test of a Differential GPS/Inertial Navigation System for Approach/Landing Guidance", Navigation: Journal of Institute of Navigation, Summer 1991, Vol. 38, No. 2, pp 103-122, incorporated herein by reference thereto.
As described therein, these differential corrections may be transmitted, by any data link technique, to a mobile unit which incorporates a GPS receiver and associated antenna experiencing substantially the same errors in the pseudo range observed values for the same tracked satellites as the ground station receiver. Accordingly, the mobile unit's pseudo range observed values may be corrected by the differential corrections for a more precise determination of the geocentric position of the mobile unit. It should be noted that the accuracy of the corrections is somewhat dependent on the proximity of the mobile unit relative to the ground station.
Although differential GPS ground stations have advanced the art of GPS for some applications, there is a need for a differential GPS ground station with enhanced integrity, continuity, and availability as particularly identified in the aforesaid RTCA publication.